The invention relates generally to image printers, and in particular to thermal printers in which the selective use of individual heating or resistive elements on a thermal print head effects a color dye transfer from a dye donor medium to a dye receiver medium to create a color image print on the dye receiver medium. More specifically, the invention provides a method and corresponding apparatus for reducing uneven use of the heating elements on the thermal print head.
A typical dye donor web that is used in a thermal printer includes a repeating series of three different primary color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent laminating section after the cyan color section.
To make a color image print using a thermal printer, respective color dyes in a single series of yellow, magenta and cyan color sections on a dye donor web are successively heat-transferred (e.g. by diffusion), one on top of the other, onto a dye receiver sheet. Then, optionally, the transparent laminating section is deposited on the color image print. The dye transfer from each color section to the dye receiver sheet is done one line of pixels at a time across the color section via a bead of selectively used heating or resistor elements on a thermal print head. The bead of heating elements makes line contact across the entire width of the dye donor web, but only those heating elements that are actually used for a particular line are heated sufficiently to effect a color dye transfer to the receiver sheet. The temperature to which a heating element is heated is proportional to the density (darkness) level of the corresponding pixel formed on the receiver sheet. The higher the temperature of the heating element, the greater the density level of the corresponding pixel. Various modes for raising the temperature of the heating element are described in prior art U.S. Pat. No. 4,745,413 issued May 17, 1988.
One example of a color print-making process using a thermal printer is as follows.
1. A dye donor web and a dye receiver sheet are advanced forward in unison, with a yellow color section of the donor web moving in contact with the receiver sheet longitudinally over a stationary bead of heating elements in order to effect a line-by-line yellow dye transfer from the yellow color section to the receiver sheet. A web take-up spool draws the dye donor web forward over the bead of heating elements, and a pair of pinch and drive rollers draw the dye receiver sheet forward over the bead of heating elements. A platen roller holds the dye receiver sheet in a dye receiving relation with the dye donor web at the bead of heating elements.
2. Once the yellow dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the pair of pinch and drive rollers to return the dye receiver sheet rearward in preparation for a second pass over the bead of heating elements.
3. Then, the platen roller is returned to adjacent the print head, and the dye donor web and the dye receiver sheet are advanced forward in unison, with a magenta color section of the donor web moving in contact with the receiver sheet longitudinally over the bead of heating elements in order to effect a line-by-line magenta dye transfer from the magenta color section to the receiver sheet. The magenta dye transfer to the dye receiver sheet is in exactly the same area on the receiver sheet as was subjected to the yellow dye transfer.
4. Once the magenta dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the pair of pinch and drive rollers to return the dye receiver sheet rearward in preparation for a third pass over the bead of heating elements.
5. Then, the platen roller is returned to adjacent the print head, and the dye donor web and the dye receiver sheet are advanced forward in unison, with a cyan color section of the donor web moving in contact with the receiver sheet longitudinally over the bead of heating elements in order to effect a line-by-line cyan dye transfer from the magenta color section to the receiver sheet. The cyan dye transfer to the dye receiver sheet is in exactly the same area on the receiver sheet as was subjected to the yellow and magenta dye transfers.
6. Once the cyan dye transfer is completed, the platen roller is retracted from adjacent the print head to allow the dye receiver sheet to be returned rearward in preparation for exiting the printer.
7. Then, the pair of pinch and drive rollers advance the dye receiver sheet forward to an exit tray.
When printing a 5xc3x977 inch color image on a 6xc3x978 inch dye receiver sheet, for example, a number of the heating elements closest to the opposite ends of the bead of heating elements are not selectively used, i.e. the heating elements closest to the opposite ends of the line are not selectively heated during the yellow, magenta and cyan dye transfers to the receiver sheet. This leaves a pair of 0.5 inch non-image (white) margin areas along opposite sides of the 5xc3x977 inch color image print on the 6xc3x978 inch receiver sheet. Alternatively, when printing a 6xc3x978 inch color image (instead of a 5xc3x977 inch image) on the 6xc3x978 inch receiver sheet, the heating elements closest to the opposite ends of the bead of heating elements are selectively used, i.e. they are selectively heated during the yellow, magenta and cyan dye transfers to the receiver sheet. As a result, a color image print without any non-image margin areas, i.e. a borderless print, is formed. If the heating elements closest to the opposite ends of the bead of heating elements are used less often than the remainder of the heating elements along the bead, there can result an uneven deterioration between the two which causes the resistance values of the two to become materially different over time. Then, when printing the 6xc3x978 inch color image, the material difference in the resistance values between a less-often-used heating element and an adjacent more-often-used heating element causes a corresponding difference in the density (darkness) levels of the dye transfer effected by the less-often-used heating element and the adjacent more-often-used heating element. As a result, an undesirable printing artifact appears as a white or gray line along the printed 6xc3x978 inch color image. This can make the color image print unacceptable.
According to one aspect of the invention, a method of reducing uneven use of the total number of printing elements on a print head in a printer, when selectively using the printing elements to make different size color image prints on respective similar size receiver mediums, comprises:
selectively using the total number of printing elements to make color image prints substantially the same size as the receiver mediums; and
selectively using a particular number of printing elements less than the total number of printing elements to make similar size color image prints smaller than the receiver mediums, but alternating which ones of the total number of printing elements can be selectively used to make each print so that the placement of each print on a receiver medium is alternated, whereby, since those printing elements that can be selectively used to make each print smaller than a receiver medium are alternated, uneven use of the printing elements is reduced.
According to another aspect of the invention an apparatus is provided for accomplishing each of the method steps.